ThermaTwin - Frequently Asked Questions

ThermaTwin is a unique flat plate solar thermal collector, which uses the power of the sun to heat the water and power the pump and controls. As a result the system is completely off-grid, and maximises the total energy available to service the needs of the home whilst reducing the carbon consumed during operation to effectively zero. Manufactured in the UK, ThermaTwin offers a robust, practical and affordable solution to your hot water and heating requirements.

According to the solar key mark tests, the ThermaTwin design in absolute terms is about 90% as efficient as many of the imported flat plate collectors and about 85% compared to an Evacuated Tube.

However these tests don’t tell the full story. Being glycol free, and thus able to run & pump using a solar pv panel and not mains electricity, the ThermaTwin system is ‘free’ to operate – something that is not measured in the performance tests. Similarly, by using the domestic in coming water direct, and thus never needing level checks or glycol tests to ensure performance and reliability, it costs practically nothing to maintain compared to other panels.

In addition the ‘bombproof’ construction means that even if a slate, tile or cricket ball hits the panel, it won’t break it; an otherwise costly repair that could wipe out a whole years savings or more with an evacuated tube for example.

Water from the bottom of the domestic hot water tank is pumped using the PV driven pump through the solar panel, as long as the panel temperature is more than 5°C hotter than the water at the bottom of the tank (to avoid the tank heating the panel and thus losing energy). This it will do, even on a cold day as long as there is daylight (and not just a hot sun) as the absorber plates in the panel absorb the suns’ UV rays irrespective of the air temperature.

The absorber plates transfer the captured heat into the water passing through the tubes sandwiched between them. The tubes are squashed oval by the absorber plates which helps them transfer more heat into the water than would be possible with a conventional round tube.

The water returns to the top of the tank providing hot water for baths and showers, without the need for an intermediate heat pump which would reduce efficiency.

The pump is designed to spin faster the more powerful the UV rays which ensures that more water is heated faster on a hot day, and less on a cloudy day, but the outgoing temperature achieved is approximately the same (about 70~75°C on average).

The variable speed pump and the fact that pure potable water is used ensures that the panel never overheats; even if the water is not being drawn off (when you’re on holiday for example) and it’s the middle of summer. Other systems are liable to boil (or stagnate) so they need a ‘drain back’ system to empty the panel if it gets too hot to avoid boiling the glycol, which would destroy it and require replacing. This added complication and control is another failure point and also means the system may not be contributing any hot water under certain conditions.

ThermaTwin, however is designed to run all day every day without damage, and in the practically impossible scenario of a full tank of hot water already at maximum temperature the system can overspill into the header tank (something that is as yet unknown with an installed system).

ThermaTwin is unique in that it uses a specially produced flexible food grade silicone tube, rather than a copper or aluminium pipe to transfer the water through the solar panel. Even though the system uses only water (glycol free) and can thus freeze, it does no damage, and as soon as the sunlight hits the panel, the absorber plates begin to thaw the water and the panel is available to produce hot water; even on a cold day (see; How does it work?).

As an added safety feature, the pump does not run if the panel is colder than the water tank, and there is a built in bypass so that even if the pump inadvertently starts when part of the panel is still frozen, the water does not over pressurise the pipework.

The simplicity of the system and the elimination of a drain-back control and anti-freeze from the water enables ThermaTwin to ‘punch above its weight’ in terms of performance compared to glycol treated systems, and the heated water to feed directly to the showers and baths if required.

Usually not. ThermaTwin is plumbed into the cold feed from the header tank from a typical vented cylinder. The pump draws the water from the tank, through the panel and back into the top of the cylinder. The domestic hot water is drawn from the top of the cylinder, so you are always drawing off the freshest, most recently heated water first. A secondary coil is not required as the water is clean untreated water and so can be pumped directly back into the system with no risk of harm to the occupant. Only if the existing cylinder is more than 10 years old, cannot be fully insulated or shows signs of wear and tear should a new cylinder be recommended.

Yes, but an intermediate heat exchanger is required and a thermal dump to ensure the unvented cylinder does not become overheated; a possibility when the property is unoccupied in summer.

It’s true that lo-iron glass would allow more light through and improve the performance of the panel. However, the additional weight and the possibility of cracking the glass due to a loose tile, slate, football, cricket ball or hail strike can cancel out a whole years energy savings if a repair is needed. Very often the panel is scrap if the lo-iron glass is damaged. The polycarbonate panel is practically bombproof and resistant to even deliberate vandal damage.

The ThermaTwin panel, despite being a 3m2 panel, weighs only 30kg dry. This makes it an easy 2 man lift, with minimal roof loading.

There are 100’s of these panels installed by DIY enthusiasts up & down the country. However to claim the RHI payments, you will need an MCS certified installation. We recommend therefore that the installation and commissioning of your ThermaTwin panel, be carried out by one of our MCS accredited installers.


DiscreteHeat Company Limited
1 Victoria Works Industrial Estate
Coal Pit Lane, Atherton
Manchester, M46 0FY

 01942 88 00 60

VAT# 884427392
Company Reg# 5594330